You can change the tyres offcourse, but I bought a complete winterset(rims + tyres). It's easier and faster to change. But I guess if(1) you buy only the tyres you're first cheaper off but in the long run(labour) I guess not...(2)

I have 19"401 summer wheels and I bought a 18"397 set Continental Wintercontact. They don't come cheap(complete € 2300 but I got a very big discount) If you are going to buy 17 or other inch you're cheaper off offcourse...

Not OEM is cheaper...but the one tyrebrand is better than the other. Continental and Pirelli are winterkings atm in EU afaik.

You live in RI? The Tirerack warehouse is in Windsor, CT. So you can save yourself $100 shipping (for tires AND wheels), by taking a ride (in something other than your 3-series ), and just picking them up. That setup was $1700 out the door (picked up myself), and after a $50 Pirelli rebate. (wheels were about $144 ea). But they look good, and the tires will give excellent performance not only on wet or snowy roads, but in dry conditions too which is important because as you know, here in New England the roads are pretty clear again a day or two after the storm. That number includes the TPM which, if you don't care about the light being on all the time, will save you another $200. But they are ready to go, so no labor cost to have someone mount and balance tires 2x per year.